In low pressure methanolysis processes for the depolymerization of polyethylene terephthalate, U.S. Pat. No. 5,051,528, a vapor stream carries the volatile reaction products away from the methanolysis reactor. The vapor stream comprises methanol, dimethyl terephthalate (DMT), methyl-2-hydroxyethyl terephthalate (MHET), ethylene glycol, other glycols, acetaldehyde and incompletely reacted polyester components. It is desirable to return as much as possible of the MHET and incompletely reacted polyester components into the reactor. This is accomplished with a fractionation device (rectifier) and heat exchanger (rectifier partial condenser), as described in U.S. Pat. No. 5,298,530. Subsequent to the depolymerization and removal of MHET and other incompletely reacted polyester components, it is desirable to recover and purify the DMT, methanol, and glycols for subsequent use or reuse.
Separating DMT from methanol and glycol, while maintaining desirable yields of DMT, is difficult because of the reactive nature of the DMT and glycol, and the existence of azeotropes. A preferred way to accomplish this separation is to crystallize the DMT in solution containing methanol prior to subsequent purification steps. For crystallization to be most effective, the feed to the crystallizer must be liquid, and the DMT concentration should be in the range of 2-40% weight to weight, with a preferred range of 20-40% weight to weight, and have a minimal amount of acetaldehyde.
The vapor from the methanolysis reactor, rectifier, and rectifier partial condenser must be condensed in order to be fed to a crystallizer. The vapor must also be fractionated because the DMT and acetaldehyde concentrations are not at the preferred concentrations for crystallization. It is preferred to accomplish both the condensation and fractionation in as few devices as possible.
When this condensation and fractionation was attempted on a laboratory scale at atmospheric pressure, and with a surface condenser, the condenser quickly fouled with DMT solids, rendering the system inoperable.